This invention relates to methods of nondestructive testing an object wherein physical surface features are enhanced for the purpose of detecting such features.
Surface features are anomalies which are placed intentionally on a surface or surface flaws or defects. It is often necessary to detect anomalies, such as, small surface defects in the form of ridges, cracks, or depressions in aggregated material, for example, diamond compacts or composite cutting tool materials. Occasionally, measurement of the surface defects is required. Some surfaces include cracks less than 1/2 mil (0.00005 inch) in width and as small as 1 micron (0.000039 inch) in width.
Various techniques have been used for detecting cracks or other small surface depressions. Some techniques use visible dye or fluorescent material carried by a penetrant applied to the surface which is to be tested. The excess is allowed to drip off and/or is shaken loose. Alternately, the surface is washed with water or other liquids or solvents may be applied to the surface. Following the removal of the excess of the liquid dye or fluorescent material, the dye or fluorescent material remains entrapped within surface cracks and makes the surface cracks stand out. Additionally, a developer may be applied to the surface to act as a blotter, thus making the crack more visible.
Instead of using dye or fluorescent material, magnetic particles have been applied to the surface so as to penetrate within those surface cracks which are large enough to receive the particles. The crack causes a non-uniformity in the magnetic field, the non-uniformity being visible from the magnetic particles.
Another method used to locate cracks or other surface flaws has been lighting the surface obliquely (i.e., the light rays being at a small angle relative to the surface) and acquiring a two-dimensional image (by a photograph, tv camera, etc.) of the surface. The existence of a crack may be inferred from detection of a shadowed area caused by the crack.
Although the above techniques of detecting cracks or other depressions in a surface have been generally useful, they have been subject to a number of limitations. Most importantly, such previous techniques have been unable to adequately detect cracks which are less than 1/2 mil in width. For example, it is difficult to guarantee that the liquid based penetrants will penetrate cracks which are less than 1/2 mil (about 12.7 microns) in width. Likewise, such cracks are generally too small for detection by magnetic flux techniques and too small to produce an adequate shadow for detection by common oblique lighting.
In addition to the aforenoted difficulties experienced by various known techniques, such prior techniques are limited in other respects. For example, the magnetic flux technique requires a ferromagnetic material. The oblique lighting or shadow technique may be difficult to work satisfactorily if the surface is in extremely dark black.
Another process measures the surface features directly by appropriate interrogating energy sources such as structured light. For example, U.S. Pat. No. 4,349,277 entitled "Non-contact Measurement of Surface Profile", issued Sept. 14, 1982, to Joseph L. Mundy, Gilbert B. Porter and Thomas M. Cipolla, assigned to the assignee of the present invention, discloses a parallax method of surface measurement based upon optical triangulation. The Mundy et al. patent uses alternate beams of different wave length which are applied to the surface of an object. Separate detector rays detect the reflected light to determine the profile of the object. Structured light is difficult to focus to sufficiently small spot sizes over a reasonable depth of field.
Accordingly, it is a primary object of the present invention to provide a new and improved method of enhancement and detection of physical features.